Waste from building demolition is found almost everywhere in streets, forests and in rivers. They are considered as a great source of nature pollution. Therefore, is can also be seen as a cause of environmental problems such as fauna and flora destruction, underground water pollution, saturation of public waste, unnecessary increase in additional sanitation water to be evacuated, nauseating garbage, insalubrity, flooding, etc. .
Recycling of such polluting waste by using it in new concrete formulation remains one of the possible solutions to that problem though Amor Ben Fraj et al., 2017 ; Wirquin et al., 2000  as well as Courard L. et al. , assumed that usage of demolition materials as substitution aggregates is sometimes very difficult to realize since such materials are porous and so highly water absorbing.
The majority of concrete from buildings demolition-based recycled aggregates are used in road construction. Nevertheless, a better knowledge of its characteristics can contribute to development of that field of construction (Buyle-Bodin et al. 2002  and Hussain et al. 2003 ). This can only be achieved by a deep knowledge of fresh concrete.
However, recycled aggregates could be used as building materials in confection of concrete when the size of the building fits with their respective characteristics. Therefore, this work is aimed to study the strength of concrete formulated by recycled aggregates in order to propose its domain of usage. In so doing, we hope to find a way of protecting the environment against the building demolition waste. The global objective of this study is to take profit from aggregates from concrete building demolition so as to perform new concrete products and contribute to environment protection by reducing building demolition waste in nature.
2. Materials and Methods
In this section, we present all tests which are necessary to describe, both qualitatively and quantitatively, the aggregates as well as the concrete. The most important characteristics for concrete were respectively their consistency and compressive strength at fresh and hardened state. The tests were conducted in the laboratory of the Université Libre des Pays des Grands Lacs. Materials that were used in this study are: aggregates (sand and gravels), water, and cement.
2.1. Origin of Constituents
Natural sand 0/5, from Lake Kivu, of density 1.36 and specific unit weight 2.5. Recycled sand 0/5, from concrete blocks fragmentation and sieving in order to obtain sand in conformity to the norm NF EN 933-1 . Natural gravels 5/25, from crushing of basaltic rock. Recycled gravels 5/25 from concrete blocks fragmentation.
Water used form concrete formulation was from “Régie de Distribution d’Eau” (REGIDESO), the water distribution company in Goma.
The cement used is the Compound Portland Cement CEM II 32.5R RHINO produced in Kenya. Its characteristics are presented in Table 1.
In this section, we present tests on aggregates on the one hand, which include particle size analysis, sand equivalent and bulk unit weight; and on concrete on the other, which include consistency and compressive strength at fresh and hardened states respectively.
2.2.1. Tests on Aggregates
1) Particle size analysis
Particle size composition is obtained by standardized particle size analysis NF EN 933-1 . Particle size analysis is aimed to determine the grains weight distribution of aggregates according to their size by plotting a particle size curve.
Particle size analysis is carried out by dividing through a series of sievers the material into different granular classes of decreasing particle size. From that segregation, we obtain the mass of particles retained by the different sievers and passing through them. They are presented in terms of their respective ratios to the initial mass. The cumulated ratio of particles passing through a given siever is obtained by the expression (1).
Table 1. Characteristics of the compound Portland cement used.
With : the mass of the particles retained by the siever (i); and the total dry of dry material sample.
For sand, from particle size analysis, we obtain the finesse module. This parameter characterizes sand size. Finesse module is obtained by the summation of the percentages of the particles retained on the different sievers of respective opennings 0.16, 0.315, 0.63, 1.25, 2.5 and 5 mm. It is preferable that the finesse module of sand for a motar and/or concrete be comprised between 2.2 and 2.8.
- A column of sievers
- Sievers vibrator
- A weighing machine
2) Cleanliness of sand
The degree of sand cleanliness is obtained by the test of sand equivalent, which is a standardized test NF EN 933-8 . The purpose of the sand equivalent test is to determine the degree of cleanliness of a sand. It defines the proportion of raw sand compared to impurities contained in the sand.
The sand equivalent is obtained by pouring a given mass of sand into two burettes containing a wash solution in order to evaluate in percentage the proportion of raw sand compared to impurities in the sand. The Sand Equivalent (SE) is calculated from the expression (2).
With h1 (cm) the height of visible sand deposit and h2 (cm) the total height (including the fines in suspension).
A washing tube
3) Specific unit weight of aggregates
The specific unit weight is obtained by the normalized test NF P 18 - 555 . The purpose of the test is to express the mass per volume unit of aggregates particles without taking into account the volume occupied by the voids.
The specific unit weight of a body is generally obtained by measuring the volume of a given liquide (generally water) that the aggregate moves when it is poured into the liquid. The results of the absolute density are calculated by the expression (3).
With Ms the mass of aggregates, the volume of water in the burette without the aggregates, the volume of water in the burette after the pouring of aggregates.
A weighing machine
2.2.2. Tests on Concrete
The consistency of concrete is obtained by the standardized test NF P 18 - 45 . The purpose of the test is to obtain the workability of the concrete so as to make its setting up and implementation easier.
The consistency is obtained by measuring the subsidence of fresh concrete under its own weight in the Abrams’ cone.
2) Compressive strength
The compressive strength of concrete is obtained by the standardized test NF P 18 - 406 . The test is aimed to determine the compressive strength of concrete.
The test consists of submitting a standardized concrete test tube to an axial compressive force up to its ruin.
3. Results and Interpretation
3.1. Results of Tests on Aggregates
3.1.1. Particle Size Analysis
The particle size composition was obtained according to the norm NF EN 933 – 1 presented previously. The results of that test are presented in Tables 2-5 and in Figure 1 for natural sand, recylcled sand, natural gravels 5/25 and recycled gravels 5/25.
From Table 2 and Table 3, we find out that the fine module of natural sand is 2.5 while that of recycled sand is 3.3. Those results show that recycled sand is very coarse since its finesse module is superior to 2.8. The recycled aggregates-based concrete has lost its workability. In order to perform its workability, we are required to increase the proportion of sand fine particles so that the finesse module be comprised between 2.2 and 2.8.
Table 2. Particle size composition of natural sand.
Table 3. Particle size composition of recycled sand.
Table 4. Particle size composition of natural gravels 5/25.
Table 5. Particle size composition of recycled du gravels 5/25.
Figure 1. Particle size curve of aggregates.
3.1.2. Sand Cleanliness
The test of sand cleanliness showed that for natural sand, the visual sand equivalent (VSE) is around 70%, while the sand equivalent to piston is 67.6% (SEP). For recycled sand, the respective values are 76.9% and 78%. These results show that the recycled sand is clean and suitable for concrete.
3.1.3. Specific unit Weight of Aggregates
Table 6 presents the different values of specific unit weight of aggregates.
Specific unit weights of recycled aggregates are inferior to those of natural aggregates. This is due to the fact that recycled aggregates, which are porous, contain some quantity of cement still bounding on the aggregates on the one hand, and the nature of the original rock of the aggregates on the other.
3.2. Results of Tets on Concretes
3.2.1. Composition of Concretes
3.2.2. Equivalent Composition
See Table 8.
3.2.3. Consistency Test
The results of subsidence test to Abrams’ cone are presented in Table 9.
The different concretes are plastic since their maneuverability is comprised in the interval between 5 cm and 9 cm. The maneuverability of reference concrete is 7 cm while that of recycled aggregates-based concrete is 9 cm. There has been observed the necessity in additional water for recycled aggregates-based concrete in order to obtain the same workability as that of the reference concrete. This shows up the need for additional water in recycled aggregates. The results presented in the Table 9 show that the formulated concretes can be used to build formwork footings, retaining walls, slabs, pavement, beams and columns.
Table 6. Specific unit weight of aggregates.
Table 7. Concrete composition (per cubic meter).
Table 8. Concrete composition on cylindrical 16 * 32 cm test-tubes.
Table 9. Results of the subsidence test.
3.2.4. Compressive Strength
The results of the compressive strength test on 16 * 32 cm cylindrical test-tubes of the different concrete samples are presented in Table 10. The concrete samples were formulated by Dreux-Gorisse Method.
In order to contribute to construction waste management and environmental protection, it has been studied the properties of concrete formulated by recycled aggregates for its usage in building. The global aim was to analyze the influence of substitution of natural aggregates by recycled aggregates from building
Table 10. Compression test results on 16 * 32 cm cylindrical test-tubes.
concrete demolition. The compressive strength of recycled aggregates-based concrete was acceptable compared to the natural aggregates-based concrete. The difference is due to the quality of structural concrete used as aggregates source. These mechanical properties show that recycled aggregates can produce concrete of strengths range between 20 and 25 MPa without using any particular sophisticated technology. Usage of recycled aggregates in concrete formulation can offer a sure and helpful solution to demolition waste management. Concrete produced from recycled aggregates is likely to be used for structures of average span and in conditions of low aggressively. However, the study of recycled concrete micro-structure and their durability can complete and improve this study. Such a study can extend the usage range of recycled aggregates-based concrete in civil engineering.
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